1. Field of the Invention
This invention relates to automatic gain control (AGC) circuits for linear amplifiers and, more specifically, to an improved temperature-compensated AGC circuit having a wide linear dynamic range.
2. Description of the Related Art
The use of automatic gain control (AGC) circuits to control amplifier gain in communication transceivers has been accomplished by various circuit designs. Typically, the dynamic range of gain control in such amplifiers is somewhat limited. This is especially true with field-effect transistor (FET) linear amplifier circuits because of the somewhat constrained linear operating region of the typical single and dual gate FET.
Amplifier gain control is also known to drift substantially with changes in circuit temperature. Temperature compensation for AGC of linear amplifiers has also been accomplished by various circuit designs. Compensation for temperature-induced drift imposes requirements different from those associated with compensation for nonlinearities in device characteristics at signal amplitude extremes.
In certain applications, such as in code division multiple access (CDMA) cellular telephone or personal communications device transceivers, where transmitter power control and receiver AGC are essential to proper system operation, the transmitter and receiver amplifiers must both track each other in gain over a relatively high dynamic signal range. In such an environment, the receiver amplifier may be required to respond linearly to a gain control signal over an 80 dB range of gain.
In U.S. Pat. No. 5,099,204, issued Mar. 24, 1992 entitled "LINEAR GAIN CONTROL AMPLIFIER" assigned to the assignee hereof, and entirely incorporated herein by this reference, Charles E. Wheatley III et al. discloses a linear gain control amplifier design having a compensation circuit that generates a compensation signal according to predetermined device characteristics. Wheatley et al compensation signal serves to linearize the nonlinear FET device characteristics at the extremes of the dynamic operating region, thereby ensuring linear amplifier gain control over a wide dynamic range. Wheatley et al. also suggests the use of a thermistor in the AGC compensation circuit to compensate for thermal drift.
In many digital communication systems, the transceiver AGC loop must provide a signal that is a logarithmic indication of a measured received signal power over a range of signal power levels. In a digital receiver, the amplified received signal power must be limited for proper signal processing of the received signal. In the cellular transceiver environment, a digital receiver may receive a signal that experiences rapid variations in signal power over a wide range. This rapid linear AGC requirement is made more difficult by the gain-tracking requirement for both receiver amplifier and transmitter amplifier in a CDMA cellular telephone transceiver. That is, in a typical digital receiver, the level of received signal power is detected, digitized and then measured. The measured value is then typically compared with a predetermined control value and a digital error signal generated. This error signal is then used to control the gain of both the receiver amplifier and the transmitter amplifier so as to adjust both received and transmitted signal strength to coincide with the respective desired signal powers. The receiver amplifier gain is carefully controlled to permit proper signal processing of the received signal. The transmitter amplifier gain is also carefully controlled to ensure sufficient signal strength in the channel without unnecessary power consumption. This demanding set of transceiver amplifier gain requirements is further exacerbated by variations in thermal drift characteristics throughout the transceiver.
Practitioners have proposed various techniques for overcoming the cellular telephone transceiver amplifier gain linearity problem. For instance, in U.S. Pat. No. 5,107,225, issued Apr. 21, 1992 entitled "HIGH DYNAMIC RANGE CLOSED LOOP AUTOMATIC GAIN CONTROL CIRCUIT" assigned to the assignee hereof and fully incorporated herein by this reference, Charles E. Wheatley, III, et al disclose a high dynamic range closed-loop AGC circuit that automatically controls transmitter and receiver amplifier gain responsive to an indication of the strength of a received signal. Wheatley, et al teach a method for combining the received signal strength indication (RSSI) with device characteristic compensation signals to provide a system wherein both the receiver and transmitter amplifier gains in decibels vary linearly with respect to a control signal over a broad dynamic range.
In these and other applications, a clear felt need is present in the art for a method for compensating amplifier gain for variations in temperature to ensure that two such amplifiers can track each other over a wide dynamic operating region. The related unresolved problems and deficiencies are clearly-felt in the art and are solved by this invention in the manner described below.